Variable low noise electrical resistor with plural variable resistors connected in series

ABSTRACT

A rheostat which includes a hollow housing having a pair of rotors rotatably mounted therein, and a resistance path on each of the rotors. One of the resistance paths provides relatively large incremental changes in resistance value over a wide range of resistance values, and the other resistance path provides small incremental changes in resistance value over a range equal to each increment of the resistance value of the one resistance path. The resistance paths are connected in series and terminals, which extend from the housing, are electrically connected to the resistance paths. By rotating the rotors, any desired resistance value over a wide range of values can be obtained. In one form of the rheostat, a separate shaft is provided to rotate each rotor. In another form of the rheostat, a single shaft drives both rotors.

United States Patent Casey et al.

[54] VARIABLE LOW NOISE ELECTRICAL RESISTOR WITH PLURAL VARIABLERESISTORS CONNECTED IN SERIES [72] Inventors: Harry B. Casey, WillowGrove, Pa.; Carl E. Clark, St. Petersburg, Fla.

[73] Assignee: TRW, Inc., Cleveland, Ohio [22] Filed: Sept. 25, 1970[21] Appl. No.: 75,638

[56] References Cited UNITED STATES PATENTS 2,972,123 2/1961 Blom..338/122X 3,027,551 3/1962 Laurin ..338/l50UX FOREIGN PATENTS ORAPPLICATIONS 844,757 5/1952 Germany ..338/123 115] 3,657,689 [45] Apr.18, 1972 Primary Examiner-Lewis H. Myers Assistant Examiner-Gerald P.Tolin Attorney-Jacob Trachtman [57] ABSTRACT A rheostat which includes ahollow housing having a pair of rotors rotatably mounted therein, and aresistance path on each of the rotors. One of the resistance pathsprovides relatively large incremental changes in resistance value over awide range of resistance values, and the other resistance path providessmall incremental changes in resistance value over a range equal to eachincrement of the resistance value of the one resistance path. Theresistance paths are connected in series and terminals, which extendfrom the housing, are electrically connected to the resistance paths. Byrotating the rotors, any desired resistance value over a wide range ofvalues can be obtained. In one form of the rheostat, a separate shaft isprovided to rotate each rotor. In another form of the rheostat, a

single shaft drives both rotors.

19 Claims, 13 Drawing Figures Patentd April 1a, 1972 3,657,689

4 Sheets-Sheet 1 2 FIG. 2 0 96d Es 15% 1 ll l? I a /4 74/ 98 a,

INVENTO/PS HARRY 8. CASE) v CARLECLARK ATTORNEY Patented April 18, 19723,657,689

4 Sheets-Sheet z 4 M 88 2 uvvavrons HARRY amuse-y CARL s. cum/r 9.2

ATTORNEY VARIABLE LOW NOISE ELECTRICAL RESISTOR WITH PLURAL VARIABLERESISTORS CONNECTED IN SERIES The present invention relates to avariable electrical resistor, and, more particularly, to a miniaturerheostat which will accurately provide any desired resistance value overa wide range of resistance values.

Rheostat type variable resistors in general comprise a path ofresistance material having a terminal at one end and a movable contactengaging the resistance material path and having a terminal connectedthereto. The contact is movable along the resistance material path tovary the resistance value between the terminals. The resistance path canbe either a wire of a metal or metal alloy wound on a substrate, or afilm of aresistance material coated on the surface of a substrate. Theresistance wires generally have asmall resistance value per unit lengthof the wire so that the movement of the contact along the wire willprovide small increments of change of the resistance of the rheostat.Although this permits the achievement of very accurate resistancevalues, the wire type resistance path has the disadvantage that itrequires a very long path to achieve high resistance values. Thus,rheostats using wire resistance paths must be relatively large toachieve high resistance values.

Film type resistance paths can have a large resistance value per unitlength so that high resistance values can be achieved with a relativelyshort length path. Thus, rheostats having film type resistance path canachieve high resistance values yet can be made small in size. However,they have the disadvantage that they do not permit the achieving of adesired resistance value with the accuracy that can be achieved with awire resistance path because of the'large resistance value per unitlength and also provide high contact resistance and contact noise.Therefore, it would be desirable to have a rheostat which combines theadvantages of both the wire and the film types of rheostats, i.e. smallsize, achievement of high resistance values and low contact resistanceand low noise, and small incremental changes for achieving accurateresistance values.

' It is therefore an object of the present invention to provide a novelrheostat with low contact resistance and low noise.

It is another object of the present invention to provide a novelrheostat which will provide accurate resistance values over a wide rangeof resistance values.

It is still another object of the present invention to provide arheostat which will provide accurate resistance values over a wide rangeof resistance values and which is small in size.

It is a further object of the present invention to provide aminiaturized rheostat which will provide accurate resistance values overa wide range of resistance values and which uses film type resistancepaths.

It is still a further object of the present invention to provide aminiaturized rheostat having two separate film type resistance paths,one of the paths providing relatively large incremental variations inresistancevalue over a wide range of resistance values, and the otherpath providingsmall incremental changes in resistance values over asmall range of resistance values.

These objects are achieved by a rheostat which includes a hollow housinghaving a pair of rotors rotatably mounted therein and being rotatablefrom outside the housing. A resistance path is on each of the rotorswith one of the resistance paths providing relatively large incrementalchanges in resistance value over a wide range of resistance values usinghighly conductive contact pads, and the other resistance path providingsmall incremental changes over a small range of low resistance valuesengaged by a slidable contact. A pair of contacts are mounted in thehousing with each contact having a terminal projecting fromthe housingand each contact being electrically connected to a separate one of theresistance paths. The resistance paths are electrically connectedtogether so that rotation of one of the rotors provides largeincremental changes in the resistance value between the terminals and inthe resistance value so as to achieve a rough and fine adjustment of theresistance value of the rheostat.

The foregoing and other objects of the invention will become moreapparent as the following detailed description of the invention is readin conjunction with the drawings, in which:

FIG. 1 is a perspective view of one embodiment of the rheostat of thepresent invention.

FIG. 2 is a top view of the rheostat with the top section of the housingremoved.

FIG. 3 is a sectional view taken along line 3-3 of FIG. 2.

FIG. 4 is a sectional view taken along line 4-4 of FIG. 3.

FIG. 5 is a sectional view taken along line 5-5 of FIG. 3.

FIG. 6 is a perspective view of the rotors and contacts of the rheostat.

FIG. 7 is a plan view of one surface of one of the rotors.

FIG. 8 is a perspective view of another embodiment of the rheostat ofthe present invention.

FIG. 9 is a back plan view of the rheostat of FIG. 8 with the back ofthe housing removed.

FIG. 10 is a front plan view of the rheostat of FIG. 8 with the frontsection of the housing removed.

FIG. 11 is a sectional view taken along line 11-11 of FIG. 10.

FIG. 12 is a sectional view taken along line 12-12 of FIG. 10.

FIG. 13 is a perspective view of the resistance paths and contacts ofthe rheostat of FIG. 8.

Like reference numerals designate likeparts throughout the severalviews.

Referring initially to FIGS. 1-7, one embodiment of the rheostat of thepresent invention is generally designated as 10. Rheostat 10 comprises ahollow, rectangular housing 12 of an electrical insulating material,such as a plastic, having mating lower and upper sections 14 and 16respectively. The lower housing section 14 has a pair of spaced,parallel semicylindrical recesses 18 and 20 extending completely acrossthe upper surface of its front wall from the inner surface to the outersurface of the front wall. The recesses 18 and 20 have enlarged diameterportions 18a and 20a respectively adjacent the inner surface of thefront wall. A notch 22 is provided in the upper surface of the frontwall of the lower housing section 1 4 between the recess 18 and theadjacent side wall of the lower housing section (see FIG. 5). The notch22 extends part way across the front wall from the inner surfacethereof. A hole 24 extends through the front wall of the lower housingsection 14 from the notch 22 to the bottom surface of the lower housingsection. A pair of spaced, parallel semicylindrical recesses 26 and 28are provided in the upper surface of the back wall of the lower housingsection 14 and are in alignment with the recesses 18 and 20respectively. The recesses 26 and 28 extend part way across the backwall from the inner surface thereof. A notch 30 is provided in the uppersurface of the back wall of the lower housing section 14 between therecess 28 and the adjacent side wall of the lower housing section (seeFIG. 4). The notch 30 extends part way across the back wall from theinner surface thereof. A hole 32 extends through the back wall of thelower housing section 14 from the notch 30 to the bottom surface of thelower housing section. A stop lug 34 projects upwardly from the bottomof the enlarged diameter portion 18a of the recess 18 (see FIG. 3). Astop lug 35 projects upwardly from the bottom of the lower housingsection 14 adjacent the back wall of the lower housing section 14 and inlongitudinal alignment with the recesses 20 and 28.

The upper housing section 16 has a pair of spaced, parallel,semicylindrical recesses 36 and 38 in the bottom surface of its frontwall, and a pair of spaced, parallel, semicylindrical recesses 40 and 42in the bottom surface of its back wall (see FIGS. 4 and 5). The recesses36 and 38 are of the same size as and are positioned to mate with therecesses 18 and 20 respectively in the lower housing section 14. Therecesses 36 and 38 have enlarged diameter portions 360 and 380 whichmate with rotation of the other rotor provides small incremental changesthe enlarged diameter portions 18a and 20a of the recesses 18 and 20.The recesses 40 and 42 in the back wall of the upper housing section 16are of the same size as and are positioned to mate with the recesses 26and 28 respectively in the back wall of the lower housing section 14.

A pair of rotor shafts 44 and 46 are rotatably mounted in ,the housing12 in spaced parallel relation. The back end of the shaft 44 is seatedand rotatably supported in the mating recesses 26 and 40 in the backwalls of the housing sections, and the front end of the shaft 44 extendsthrough and is rotatably supported in the mating recesses 18 and 36 inthe front walls of the housing sections. The back end of the shaft 46 isseated and rotatably supported in the mating recesses 28 and 42 in theback walls of the housing sections, and the front end of the shaft 46extends through and is rotatably supported in the mating recesses and 38in the front walls of the housing sections. Enlarged heads 48 and 50 areprovided on the front ends of the shafts 44 and 46 respectively, and areoutside of the housing 12 so as to permit rotation of the shafts. Theheads 48 and 50 have slots 52 and 54 respectively extending across theirfront surfaces. The slots 52 and 54 are adapted to receive a screwdriver or the like tool for rotating the shafts. Annular flanges 56 and58 extend radially outwardly from the shafts 44 and 46 respectivelyadjacent their front ends. The flange 56 fits in the mating enlargeddiameter portions 180 and 36a of the recesses 18 and 36 to preventlongitudinal movement of the shaft 44. The flange 58 fits in the matingenlarged diameter portions 20a and 38a of the recesses 20 and 38 toprevent longitudinal movement of the shaft 46. The

Rotor 66 has a stop lug 84 projecting radially from its peripheral edge.The stop lug 84 is adapted to engage the stop lug 35 of the housing 12to limit the degree of rotation of the rotor 66 (see FIG. 3). A secondresistance path 86 is provided on the back surface of the rotor 66 (seeFIG. 7). The resistance path 86 has spaced ends with one of the endsbeing positioned adjacent a side of the stop lug 84. The resistance path86 is a resistance material coated on the back surface of the rotor 66.A termination film 88 of an electrically conductive material is coatedon the rotor 66. The termination film 88 extends from the end of theresistance path which is adjacent the side of the stop lug 84 over theperipheral edge of the rotor 66 onto the front surface of the rotor.

An insulated wire 90 electrically connects the resistance path 80 andthe resistance path 86 in series. One end of the wire 90 is soldered tothe termination film 82 at the back surface of the rotor 64, and theother end of the wire is soldered to the termination film 88 at thefront surface of the rotor 66. The wire 90 extends one turn around eachof the shafts 44 and 46. Thus, one end of the resistance path 80 iselectrically connected to an end of the resistance path 86. A detentspring member 92 is mounted on the bottom of the lower housing section14 and extends longitudinally along and under the shaft 44. The detentspring member 92 is bent upwardly to the edge of the rotor 64 toprovidea lug 94 which extends into the notches 75 in the rotor 64.

A pair of contact members 96 and 98 are mounted in the housing 12. Thecontact members 96 and 98 are metal strips shafts 44 and 46 have squareportions 60 and 62 respectively having top sections 96a and 98arespectively, arcuate arms which are within the housing 12. The squareportion 60 of the shaft 44 is adjacent the front wall of the housing,and the square portion 62 of the shaft 46 is adjacent the back wall ofthe housing. A stop lug 63 projects from the shaft 44 within theenlarged diameter portions 180 and 36a of the recesses 18 and 36, and isadapted to engage the stop lug 34 to limit the degree of rotation of theshaft 44. A separate seal ring, not shown, may be provided around eachshaft 44 and 46 between the shafts and the front walls of the housingsections 14 and 16.

- Rotors 64 and 66 are mounted on the shaft 44 and 46 respectivelywithin the housing 12. The rotors 64 and 66 are each a flat, circulardisk of an electrical insulating material, preferably a material whichwill withstand high temperatures, such as a ceramic. The rotors 64 and66 have square holes 68 and 70 respectively through the centers thereofwhich receive the square portions 60 and 62 of the shafts 44 and 46 sothat the rotors will rotate with the shafts. Snap rings 72 and 74 extendaround the shafts 44 and 46 respectively adjacent the rotors 64 and 66to hold the rotors on the shafts.

Rotor 64 has a plurality of notches 75 in its peripheral edge which areuniformly spaced around the edge of the rotor (see FIGS. 3 and 6). Aplurality of spaced contact pads 76 of an electrically conductivematerial are coated on the front surface of the rotor 64 adjacent theperipheral edge of the rotor. The contact pads 76 are arranged along acircular path with each contact pad being in radial alignment with aseparate one of the notches 75 in the rotor. The number of the contactpads is equal to the number of the notches 75. A separate connectingstrip 78 of an electrically conductive material extends radiallyinwardly along the front surface of the rotor 64 from each of thecontact pads 76. An annular resistance path 80 is on the front surfaceof the rotor 64 and extends over the inner ends of the connecting strips78. The resistance path 80 has spaced ends which are at a pair ofadjacent connecting strips 780 and 78b (see FIG. 6). The resistance path80 is a film of a resistance material, such as a metal, mixture ofmetals, metal alloy, or particles of an electrically conductive materialembedded in a matrix of glass or plastic, coated on the surface of therotor. A termination film 82 of an electrically conductive material iscoated on the rotor 64 and extends fromthe contact pad 760 at the end ofthe connecting strip 78a across the notch adjacent the contact pad 76aand onto the back surface of the rotor 64.

96b and 98b extending from one end of the top sections, a plurality ofparallel fingers 96c and 980 on the ends of the arms 96b and 98b, andterminals 96d and 98d extending from the other ends of the top sections.The contact member 96 is mounted on the front wall of the lower housingsection 14 with the top section 96a being seated in the notch 22 and theterminal 96d extending through the hole '24 and projecting beyond thebottom of the housing 12. The arm 96b of the contact member 96 extendsdownwardly along the front surface of the rotor 64 with the fingers 96cslidably engaging the front surface of the rotor at a point directlyover the detent spring member 92. The fingers 96c engage the surface ofthe rotor 64 at the circular path of the contact pads 76 so that thefingers engage the contact pads 76 as the rotor 64 rotates. The contactmember 98 is mounted on the backwall of the lower housing section 14with the top section 98a being seated in the notch 30 and the terminal98d extending through the hole 32 and projecting beyond the bottom ofthe housing 12. The arm 98b of the contact number 98 extends downwardlyalong the back surface of the rotor 66 and the fingers 98c slidablyengage the resistance path 86.

The terminal 96d is electrically connected to the resistance path by thecontact pads 76 and their connecting strips 78. As the shaft 44 isrotated, the contact fingers 96c engage different ones of the contactpads 76 so as to change the position along the resistance path to whichthe terminal 96d is connected. As the shaft'44 is rotated, the detentspring lug 94 snaps out of and into the notches 75 in the rotor 64 so asto indicate when the contact fingers 960 are engaging a contact pad 76.The terminal 98d is electrically connected to the resistance path 86 bythe contact fingers 98c. As the shaft 46 is rotated, the terminal 98d isconnected to different points along the resistance path 86.

The terminal 96d is electrically connected to the terminal 98d throughthe contact 96, resistance path 80, connecting wire 90, resistance path86 and contact 98. The resistance value measured between the terminals96d and 98d is the resistance value of the portion of the resistancepath 80 between the contact pad 76 engaged by the contact fingers 96cand the contact pad 76a plus the resistance value of the portion of theresistance path 86 between the contact fingers 98c and the terminationfilm 88. By rotating one or both of the shaft 44 and 46, the resistancevalue measured between the terminals 96d and 98d is varied. Theresistance path 80 is of a resistance material having a relatively highresistivity so that the resistance of each incremental length of theresistance path 80 between adjacent connecting strips 78 is relativelyhigh. The resistance path 86 is of a resistance material having a lowerresistivity and is selected such that the total resistance of theresistance path 86 is equal to the resistance value of each incrementallength of the resistance path 80. For example, if each incrementallength of the resistance path 80 is of a resistance value of 100 ohms,and the resistance path 80 has nine increments, the total resistancevalue of the resistance path 80 would be 900 ohms. The resistance path86 would then be made to have a total resistance value of 100 ohms.

If the shaft 44 is rotated to place the contact fingers 960 on thecontact pad 76a, and the shaft 46 is rotated to place the contactfingers 98c at the termination film 88, the resistance value between theterminals 96d and 98d would be zero. By rotating the shaft 46 so as tomove the contact fingers 98c along the resistance path 86, theresistance value measured between the tenninals 96d and 98d would varyin small increments from zero to 100 ohms. If the shaft 44 is rotated toplace the contact fingers 960 on the contact pad 76 next adjacent thecontact pad 76a, rotation of the shaft 46 would vary the resistancevalue between the terminals 96d and 98d in small increments between 100ohms and 200 ohms. As the shaft 44 is rotated to move the contactfingers 96c consecutively from one contact pad 76 to the next, theresistance value of the rheostat increases by 100 ohms. Rotation of theshaft 46 moves the contact fingers 98c along the resistance path 86 soas to vary the resistance value of the rheostat 10 in small incrementsbetween the incremental change provided by the resistance path 80. Thus,rotation of the shaft 44 provides a coarse setting of the resistancevalue of the rheostat 10, and rotation of the shaft 46 provides a finesetting of the resistance value. The resistance path 80 providesincremental changes of resistance value over a wide range of resistancevalues, and the resistance path 86 provides small incremental changesover a narrow range of values, the range of each incremental change ofthe resistance path 80. This provides a rheostat 10 which provides smallincremental changes in resistance value over a wide range of values soas to permit the accurate adjustment of the rheostat 10 to any desiredresistance value over the wide range of values. In addition, since therheostat 10 uses film type resistance paths, the rheostat can be madesmall in size and still provide the wide range of resistance values.

Referring to FIGS. 8-13 inclusive, another embodiment of the rheostat ofthe present invention is generally designated as 100. The rheostat 100comprises a hollow rectangular housing 102 of an electrical insulatingmaterial, such as a plastic, having mating front and back sections 104and 106. The front sec tion 104 has a circular hole 108 through itsfront wall at one side of the center of the front wall, and a circularbearing hub 110 (see FIG. 11) projecting from the inner surface of thefront wall at the other side of the center of the front wall. The backsection 106 has a hub 112 (See FIG. 12) projecting from the innersurface of its back wall in alignment with the hole 108 in the frontsection 104. The hub 112 has a groove 114 extending diagonallythereacross (see FIG. 10).

A rotor 116, which is a flat, circular disk of an electrical insulatingmaterial, is within the housing 102 between the hole 108 in the frontsection 104 and the hub 112 on the back section 106. A rotor shaft 118is either secured to or integral with the center of the front surface ofthe rotor 116. The shaft 118 extends through and is rotatably supportedin the hole 108 in the front housing section 104. A slot 120 extendsacross the front end of the shaft and is adapted to receive a screwdriver or like tool to rotate the shaft. A sealing ring may be providedaround the rotor shaft 118 between the shaft and the front wall of thehousing so as to seal the hole 108. The hub 112engages the back surfaceof the rotor 116 to hold the rotor in position with the shaft extendingthrough the hole 108. A drive lug 122 extends radially outwardly fromthe shaft 118 (see FIG. 10) within the housing 102 and adjacent thefront surface of the rotor 116. An annular resistance path 124 havingspaced ends is coated on the back surface of the rotor l 16 (see FIGS. 9and 13) and extends around the peripheral edge of the back surface. Theresistance path 124 may be of any type of resistance material. Acircular contact path 126 is on the center of the back surface'of therotor 116 within the resistance path 124 and is electrically connectedto one end of the resistance path by a termination strip 128. Thecontact path 126 and termination strip 128 are films of an electricallyconductive material coated on the surface of the rotor 116.

A second rotor 130, which is a flat, circular disk of anelectricalinsulating material, is within the housing 102 adjacent thebearing hub 110. A hub shaft 132 is secured to or integral with thefront surface of the rotor 130. The hub shafi 132 has a circular hole134 (see FIG. 10) in its front end which receives the bearing hub sothat the hub shaft 132 and the rotor are rotatably supported on thebearing hub 110. The rotor 130 has a plurality of circumferentiallyspaced notches 136 in its peripheral edge. The rotor 130 also has acircumferentially elongated groove 138 in its peripheral edge in thespace between two of the notches 136. The rotor 130 overlaps a portionof the front surface of the rotor 116 to the extend that the drive lug122 on the shaft 118 extends into one of the notches 136 in the rotor130 (see FIG. 10). The outer surface of the hub shaft 132 is serrated bya plurality of V-shaped grooves 140. The bottom of each of the grooves140 is in radial alignment with a separate one of the grooves 136 in therotor 130 (see FIG. 10). A V-shaped spring member 141 is compressedbetween the hub shaft 132 and the side wall of the front housing section104 with the apex of the spring member fitting in a groove 140 in thehub shaft (see FIG. 10).

An annular resistance path 142 having spaced ends is on the back surfaceof the rotor 130 and extends around the rotor adjacent the peripherythereof (see FIG. 9). The resistance path 142 is of any desiredresistance material coated on the back surface of the rotor 130. Theends of the resistance path 142 are positioned in alignment with thenotches 136a and 1361; which are at opposite ends of the groove 138. A-circular contact path 144 is on the center of the back surface of therotor 130 and is electrically connected to one end 1420 of theresistance path 142 by a tennination strip 146 which extends over theend 1420 of the resistance path (see FIGS. 9 and 13). The contact path144 and the termination strip 146 are films of an electricallyconductive material. A plurality of circumferentially spaced contactpads 148 are provided on the back surface of the rotor 130 in the spacebetween the resistance path 142 and the contact path 144. Each of thecontact pads 148 is electrically connected to the resistance path 142 bya connecting strip 150 which extends radially from the contact pad overthe resistance path. Each connecting strip 150 is tapered in width withits narrowest end being at it respective contact pad 148. The narrow endof the connecting strip is narrower than the width of the contact pad.Each contact pad 148 and its respective connecting strip 150 is inradial alignment with a separate one of the notches 136 in the edge ofthe rotor 130 so that the contact pads 148 are electrically connected tocircumferentially spaced points along the resistance path 142. Thus oneof the contact pads 148 is electrically connected to the end 142b of theresistance path 142. The contact pads 148 and the connecting strips 150are films of an electrically conductive metal coated on the surface ofthe rotor 130.

A pair of metal strip contacts 152 and 154 are within the housing 102between the rotors 116 and 130 and the back housing section 106. Each ofthe contacts 152 and 154 is bent over at its upper end to provide acontact arm 152a and 154a respectively extending toward the rotors 116and 130. Each of the contact arms 152a and 1540 has a pair of contactfingers l52b and 154b respectively on its end. Terminals 152C and 154Cextend downwardly from the bottom ends of the contacts 152 and 154respectively. The contact 152 is seated in the groove 114 in the hub 112on the back housing section 106 with the contact fingers 152b slidablyengaging the contact path 126 on the rotor 116. The terminal 152cextends through a notch 156 in the bottom of the back housing section106 and projects beyond the bottom of the housing 102. The contact 154is seated behind the rotor 130 with the contact fingers 15412 slidablyengaging the contact path 144. The terminal l54c extends through a notch158 in the bottom of the back section 106 and projects beyond the bottomof the housing A flat, metal connecting strip 160 extends along and isseated in a groove 162 in the bottom of the front housing section 104(see FIGS. 9, 11 and 12). A pair of connecting arms 164 and 166 areintegral with and extend upwardly from the connecting strip 160. Each ofthe connecting arms 164 and 166 has a plurality of fingers 164a and 166arespectively on its upper end. The fingers 164a slidably engage theresistance path 124 on the rotor 116, and the fingers 166a slidablyengage the contact pads 148 and connecting strips 150 on the rotor 130.Thus, the connecting strip 160 and its arms 164 and 166 electricallyconnect the resistance paths 124 and 142 so as to electrically connectthe terminal 152s to the terminal 1540 through the resistance paths.

The resistance path 142 on the rotor 130 is of a resistance materialhaving a relatively high resistivity so that the resistance of eachincremental length of the resistance path 142 between adjacentconnecting strips 150 is relatively high. By rotating the rotor 130, theconnecting arm fingers 166a slide over the rotor from one set of contactpad 148 and connecting strip 150 to the next set so that the resistancevalue between the terminal 154C and the connecting arm 166 varies inrelatively large incremental steps. The resistance path 124 on the rotor116 is of a resistance material having a low resistivity such that theresistance value of the entire length of the resistance path 124 isequal to the resistance value of each incremental step of the resistancepath 142 on the rotor 130. Since the connecting arm fingers 164adirectly contact the resistance path 124, as the rotor 116 is rotated,the resistance value between the terminal 152a and the connecting arm164 varies in very small increments. Thus, rotation of the rotor 130provides large incremental changes in resistance value over a largerange of resistance values, and rotation of the rotor 116 provides smallincremental changes over a resistance range equal to each incrementalchange of the resistance path 142 on the rotor 130. Since the resistancevalue between the terminals 152s and 1546 is equal to the resistancevalue between the terminal 152a and the connecting arm 164 plus theresistance value between the terminal'154c and the connecting arm 166,rotation of the two rotors 116 and 130 can provide any desiredresistance value over a wide range of resistance values.

The rotor 116 is rotated by rotating the shaft 118. When the rotor 116is rotated so that the connecting arm fingers 164a are at either end ofthe resistance path 124, the drive lug 122 on the shaft 118 extends intoa notch 136 in the edge of the rotor 130 (see FIG. 10. Further rotationof the shaft 118 in the same direction causes the drive lug 122 torotate the rotor 130 until the drive lug moves out of the notch 136.This rotates the rotor 130 a circumferential distance equal to thedistance between the notches 130. Since the contact pads 148 are inradial alignment with the notches 130, each such rotation of the rotor130 moves the connecting arm fingers 166a from one contact pad 148 andits related connecting strip 150 to the next. Thus, each time that therotor 116 is rotated so that the connecting arm fingers 164a pass acrossthe space between the ends of the resistance path 124, the rotor 130 isrotated a distance to move the contact arm fingers 166a from one contactpad 148 to the next so as to change the resistance value between theterminals 152s and 1540 by one incremental change of the resistance path142. If the rotor 116 is rotated to move the connecting arm fingers 164afrom its high resistance end to its low end, the rotor 130 is rotated todecrease the resistance value provided by the resistance path 142 andvice versa. Thus, the single shaft 118 rotates both rotors 116 and 130so that the rotor 130 is rotated in incremental steps to provideincremental changes in resistance value over a wide range of resistancevalues and the rotor 116 is rotated to provide small incremental changesin resistance value over the range of each incremental change providedby the resistance path 142 on the rotor 130.

When the rotor is not being rotated by the drive lug 122, it isprevented from accidental rotation by the spring 141 fitting in one ofthe notches in the hub shaft 132. When the rotor 130 is rotated by thedrive lug 122, thespn'ng 141 snaps from one notch 140 to the next. Whenthe rotor 130 is rotated to a position where the connecting arm fingers166a contact the connecting strip at either end of the resistance path,the next complete rotation of the shaft 118 brings the drive lug 122into the groove 138 in the rotor 130. Since the groove 138 is much widerthan the width of the drive lug 122, further rotation of the shaft 118will not cause any rotation of the rotor 130. Thus, the groove 138 actsas a clutch to prevent the connecting arm fingers 166a from being movedacross the spaced between the ends of the resistance path 142. When therotor 130 is rotated to shift the connecting arm fingers 166a from onecontact pad 148 and its related connecting strip 150 to the finger thefingers at one side of the arm moves to the next contact pad orconnecting strip before the finger at the other side of the arm leavesthe one contact pad or connecting strip because of the tapered shape ofthe connecting strips. Thus, there is a smooth transition in the changeof the resistance value without any breaks in the circuit through therheostat.

The variable resistors 10 and 100 have their housing sections joined toeach other and to the terminals extending.

therethrough by suitable bonding techniques.

The disclosed devices thus provide accurate resistance values over awide range while being small in size. Also since the series contactresistances are a function of the resistances of the materialscontacted, the variable resistors 10 and 100 by utilizing highlyconductive pads for engagement by respective contact fingers, in serieswith respective low resistance paths (-86, 124) for slidable engagementby respective fingers (98, 164), maintain their total series contactresistances at low values. Because of such low contact resistance, thenoise generated by the low resistor paths (86, 124) during theiradjustment is also low. The resistors thus provide infinite resolutionwith low noise over a wide range of resistance adjustment.

It will, of course, be understood that the description and drawings,herein contained, are illustrative merely and that various modificationsand changes may be made in the structures disclosed without departingfrom the spirit of the invention.

What is claimed is:

1. A variable resistor comprising a hollow housing;

a pair of rotors rotatably mounted in said housing;

means for rotating each of said rotors from outside of said housing;

a separate resistance path on each of said rotors, each of saidresistance paths being a film of a resistance material coated on therotor, the resistance path on oneof the rotors being of a highresistivity resistance material so as to provide large incrementalchanges in resistance along its length and the resistance path on theother rotor being of lower resistivity material so as to provide smallincremental changes in resistance along its length,

the one rotor having the film of high resistivity resistance materialhas a plurality of circumferentially spaced contact pads of anelectrically conductive material each electrically connected to theresistance path at spaced points along the resistance path so as todivide said resistance paths into incremental lengths of resistance;

a pair of contact within said housing, one of said contact slidablyengaging the contact pads of said one rotor as the one rotor is rotated,and the other contact slidably engaging the resistance path on the otherrotor and means electrically connecting said resistance paths to eachother;

whereby said contact which slidably engaging the contact pads providesslow contact resistance and low noise when incremental resistancevariations are made along the high resistivity film.

2. A variable resistor in accordance with claim 1 in which each of therotors is a flat circular disk of an electrical insulating material andthe resistance paths are annular with spaced ends and extendcircumferentially around a flat surface of their respective rotors.

3. A variable resistor in accordance with claim 2 in which the one rotorhas the plurality of circumferentially spaced contact pads of anelectrically conductive material on the same surface as the resistancepath and a separate connecting strip of an electrically conductivematerial extends from each of said contact pads to the resistance path,said connecting strips contacting said resistance path at uniformlyspaced points along the resistance path so as to divide said resistancepath into incremental lengths of uniform resistance.

4. A variable resistor in accordance with claim 3 in which theresistance value of the total length of the resistance path on the otherrotor is substantially equal to the resistance value of each incrementallength of the resistance path on the one rotor.

5. A variable resistor in accordance with claim 4 in which each of thecontacts is a metal strip having an arm extending from one end, fingerson the end of the arm, and the terminal extending from the other end,the fingers of each of the contacts slidably engages a separate one ofthe rotors.

6. A variable resistor in accordance with claim 5 in which each of therotors has on the same surface as the resistance path a contact path ofan electrical conductive material and a termination strip of anelectrically conductive material extending between the contact path andone end of the resistance path, and the fingers of each of the contactsengages a separate one of the contact paths.

7. A variable resistor in accordance with claim 8 in which the meanselectrically connecting the resistance paths comprises a connectingstrip of an electrically conductive metal mounted in the housing andhaving a pair of arms extending therefrom with fingers on the ends ofthe arms, the fingers on one of the arms slidably engage the one rotorso as to contact the contact pads and connecting strips as the rotor isrotated, and the fingers on the other arm slidably engage the resistancepath on the other rotor.

8. A variable resistor in accordance with claim 7 in which the means forrotating the rotors comprises a shaft secured to the other rotor andextending through a wall of the housing so as to be accessible fromoutside the housing, and drive means between the shaft and the one rotorso that the one rotor is rotated a circumferential distance equal to thecircumferential length of each increment of the resistance path on theone rotor each time the shaft is rotated through a revolution.

9. A variable resistor comprising a hollow housing; a pair of rotorsrotatably mounted in said housing; means for rotating each of saidrotors from outside of said housing; a separate resistance path on eachof said rotors, each of said resistance paths being a film of aresistance material coated on the rotor, the resistance path on one ofthe rotors being of a high resistivity resistance material so as toprovide large incremental changes inresistance along its length and theresistance path on the other rotor being of lower resistivity materialso as to provide small incremental changes in resistance along itslength; a pair of contacts within said housing, each of said contactshaving a terminal extending through and projecting beyond a wall of saidhousing; means electrically connecting each of said contacts toaseparate one of said resistance paths; and means electricallyconnecting said resistance paths to each other; each of the rotors beinga flat circular disk of an electrical insulating material and theresistance paths being annular with spaced ends and extendingcircumferentially around a fiat surface of their respective rotors; theone rotor having a plurality of circumferentially spaced contact pads ofan electrically conductive material on the same surface as theresistance path and a separate connecting strip of an electri callyconductive material extending from each of said contact pads to theresistance path, said connecting strips contacting said resistance pathat uniformly spaced points along the resistance path so as to dividesaid resistance path into incremental lengths of unifomi resistance; theresistance value of the total length of the resistance path on the otherrotor being substantially equal to the resistance value of eachincremental length of the resistance path on the one rotor; each of thecontacts being a metal strip having an arm extending from one end,fingers on the end of the arm, and the terminal extending from the otherend, the fingers of each of the contacts slidably engaging a separateone of the rotors; the fingers of one of the contacts engaging thesurface of the one rotor so as to contact the contact pads as the onerotor is rotated, and the fingers of the other contact slidably engagingthe resistance path on the other rotor.

10. A variable resistor in accordance with claim 9 in which the meanselectrically connecting the resistance paths comprises a wireelectrically connected at one end to one end of one of the resistancepaths and at its other end to one end of the other resistance path.

11. A variable resistor in accordance with claim 10 in which the meansfor rotating each of the rotors comprises a separate shaft rotatablysupported on the housing and projecting through a wall of the housing soas to be accessible from outside the housing.

12. A variable resistor in accordance with claim 11 in which the onerotor has a plurality of circumferentially spaced notches in its outeredge, each of said notches being in radial alignment with a separate oneof the contact pads on the one rotor, and a detent spring member ismounted in the housing and extends into one of the notches in theonerotor, said detent spring member being adapted to move out of and intothe notches as the one rotor is rotated.

13. A variable resistor in accordance with claim 12 in which the housingis rectangular having front, back, top, bottom and side walls, theshafts extend in spaced parallel relation between and are supported onthe front and back walls and project through the front wall of the ofthe housing, and each of the rotors is mounted on a separate one of theshafts.

14. A variable resistor in accordance with claim 13 in which theresistance path on the one rotor is on the front surface of the onerotor, the one contact is mounted on the front wall of the housing withits terminal extending downwardly through the front wall and projectingbeyond the bottom wall of the housing, the resistance path on the otherrotor is on the back surface of the rotor and the other contact ismounted on the back wall with its terminal extending downwardly throughthe back wall and projecting beyond the bottom wall of the housmg.

15. A variable resistor in accordance with claim 11 including stop meansfor limiting the rotation of each of the rotors to the circumferentiallength of the resistance path on the rotor.

16. A variable resistor comprising a hollow housing; a pair of rotorsrotatably mounted in said housing; means for rotating each of saidrotors from outside of said housing; a separate resistance path on eachof said rotors, each of said resistance paths being a film of aresistance material coated on the rotor, the resistance path on one ofthe rotors being of a high resistivity resistance material so as toprovide large incremental changes in resistance along its length and theresistance path on the other rotor being of lower resistivity materialso as to provide small incremental changes in resistance along itslength; a pair of contacts within said housing, each of said contactshaving a terminal extending through and projecting beyond a wall of saidhousing; means electrically connecting each of said contacts to aseparate one of said resistance path; and means electrically connectingsaid resistance path to each other; each of the rotors being a fiatcircular disk of an electrical insulating material and the resistancepaths being annular with spaced ends and extending circumferentiallyaround a flat surface of their respective rotors; the one rotor having aplurality of circumferentially spaced contact pads of an electricallyconductive material on the same surface as the resistance path and aseparate connecting strip of an electrically conductive materialextending from each of said contact pads to the resistance path, saidconnecting strips contacting said resistance path at uniformly spacedpoints along the resistance path so as to divide said resistance pathinto incremental lengths of uniform resistance; the resistance value ofthe total length of the resistance path on the other rotor beingsubstantially equal to the resistance value of each incremental lengthof the resistance path on the one rotor; each of the contacts being ametal strip having an arm extending from one end, fingers on the end ofthe arm, and the terminal extending from the other end, the fingers ofeach of the contacts slidably engaging a separate one of the rotors;each of the rotors having one the same surface as the resistance path acontact path of an electrical conductive material and a terminationstrip of an electrically conductive material extending between thecontact path and one end of the resistance path, and the fingers of eachof the contacts engaging a separate one of the contact paths; the meanselectrically connecting the resistance paths comprising a connectingstrip of an electrically conductive metal mounted in the housing andhaving a pair of arms extending therefrom with fingers on the ends ofthe arms, the fingers on one of the arms slidably engaging the one rotorso as to contact the contact pads and connecting strips as the rotor isrotated, and the fingers on the other arm slidably engaging theresistance path on the other rotor; the means for rotating the rotorscomprising a shaft secured to the other rotor and extending through awall of the housing so as to be accessible from outside the housing, anddrive means between the shaft and the one rotor so that the one rotor isrotated a circumferential distance equal to the circumferential lengthof each increment of the resistance path on the one rotor each time theshaft is rotated through a revolution; the drive means between the shaftand the rotor comprising a plurality of circumferentially spaced notchesin the edge of the one rotor, each of said notches being in radialalignment with a separate one of the contact pads on the one rotor, anda drive lug extending radially from the shaft, said drive lug fittinginto and driving the one rotor during a portion of each revolution ofrotation of the shaft.

17. A variable resistor in accordance with claim 16 in which the onerotor has a circumferentially elongated groove in its edge extendingalong the space between the ends of the resistance path on the one rotorand between the notches at the ends of the resistance path.

18. A variable resistor in accordance with claim 17 including a hubshaft secured to the one rotor and a bearing hub on the same wall of thehousing that the shaft extends through, said bearing hub rotatablysupporting the hub shaft so that the rotors rotate about spaced parallelaxes.

19. A variable resistor in accordance with claim 18 in which the hubshaft has a plurality of circumferentially spaced notches in its outersurface with the bottom of each notch being in radial alignment with acontact pad on the one rotor, and a spring member fits in a notch in thehub shaft and is adapted to move from one notch to the next as the hubshaft is rotated.

1. A variable resistor comprising a hollow housing; a pair of rotorsrotatably mounted in said housing; means for rotating each of saidrotors from outside of said housing; a separate resistance path on eachof said rotors, each of said resistance paths being a film of aresistance material coated on the rotor, the resistance path on one ofthe rotors being of a high resistivity resistance material so as toprovide large incremental changes in resistance along its length and theresistance path on the other rotor being of lower resistivity materialso as to provide small incremental changes in resistance along itslength; the one rotor having the film of high resistivity resistancematerial has a plurality of circumferentially spaced contact pads of anelectrically conductive material each electrically connected to theresistance path at spaced points along the resistance path so as todivide said resistance paths into incremental lengths of resistance; apair of contact within said housing, one of said contact slidablyengaging the contact pads of said one rotor as the one rotor is rotated,and the other contact slidably engaging the resistance path on the otherrotor and means electrically connecting said resistance paths to eachother; whereby said contact which slidably engaging the contact padsprovides low contact resistance and low noise when incrementalresistance variations are made along the high resistivity film.
 2. Avariable resistor in accordance with claim 1 in which each of the rotorsis a flat circular disk of an electrical insulating material and theresistance paths are annular with spaced ends and extendcircumferentially around a flat surface of their respective rotors.
 3. Avariable resistor in accordance with claim 2 in which the one rotor hasthe plurality of circumferentially spaced contact pads of anelectrically conductive material on the same surface as the resistancepath and a separate connecting strip of an electrically conductivematerial extends from each of said contact pads to the resistance path,said connecting strips contacting said resistance path at uniformlyspaced points along the resistance path so as to divide said resistancepath into incremental lengths of uniform resistance.
 4. A variableresistor in accordance with claim 3 in which the resistance value of thetotal length of the resistance path on the other rotor is substantiallyequal to the resistance value of each incremental length of theresistance path on the one rotor.
 5. A variable resistor in accordancewith claim 4 in which each of the contacts is a metal strip having anarm extending from one end, fingers on the end of the arm, and theterminal extending from the other end, the fingers of each of thecontacts slidably engages a separate one of the rotors.
 6. A variableresistor in accordance with claim 5 in which each of the rotors has onthe same surface as the resistance path a contact path of an electricalconductive material and a termination strip of an electricallyconductive material extending between the contact path and one end ofthe resistance path, and the fingers of each of the contacts engages aseparate one of the contact paths.
 7. A variable resistor in accordancewith claim 8 in which the means electrically connecting the resistancepaths comprises a connecting strip of an electrically conductive metalmounted in the housing and having a pair of arms extending therefromwith fingers on the ends of the arms, the fingers on one of the armsslidably engage the one rotor so as to contact the contact pads andconnecting strips as the rotor is rotated, and the fingers on the otherarm slidably engage the resistance path on the other rotor.
 8. Avariable resistor in accordance with claim 7 in which the means forrotating the rotors comprises a shaft secured to the other rotor andextending through a wall of the housing so as to be accessible fromoutside the housing, and drive means between the shaft and the one rotorso that the one rotor is rotated a circumferential distance equal to thecircumferential length of each increment of the resistance path on theone rotor each time the shaft is rotated through a revolution.
 9. Avariable resistor comprising a hollow housing; a pair of rotorsrotatably mounted in said housing; means for rotating each of saidrotors from outside of said housing; a separate resistance path on eachof said rotors, each of said resistance paths being a film of aresistance material coated on the rotor, the resistance path on one ofthe rotors being of a high resistivity resistance material so as toprovide large incremental changes in resistance along its length and theresistance path on the other rotor being of lower resistivity materialso as to provide small incremental changes in resistance along itslength; a pair of contacts within said housing, each of said contactshaving a terminal extending through and projecting beyond a wall of saidhousing; means electrically connecting each of said contacts to aseparate one of said resistance paths; and means electrically connectingsaid resistance paths to each other; each of the rotors being a flatcircular disk of an electrical insulating material and the resistancepaths being annular with spaced ends and extending circumferentiallyaround a flat surface of their respective rotors; the one rotor having aplurality of circumferentially spaced contact pads of an electricallyconductive material on the same surface as the resistance path and aseparate connecting strip of an electrically conductive materialextending from each of said contact pads to the resistance path, saidconnecting strips contacting said resistance path at uniformly spacedpoints along the resistance path so as to divide said resistance pathinto incremental lengths of uniform resistance; the resistance value ofthe total length of the resistance path on the other rotor beingsubstantially equal to the resistance value of each incremental lengthof the resistance path on the one rotor; each of the contacts being ametal strip having an arm extending from one end, fingers on the end ofthe arm, and the terminal extending from the other end, the fingers ofeach of the contacts slidably engaging a separate one of the rotors; thefingers of one of the contacts engaging the surface of the one rotor soas to contact the contact pads as the one rotor is rotated, and thefingers of the other contact slidably engaging the resistance path onthe other rotor.
 10. A variable resistor in accordance with claim 9 inwhich the means electrically connecting the resistance paths comprises awire electrically connected at one end to one end of one of theresistance paths and at its other end to one end of the other resistancepath.
 11. A variable resistor in accordance with claim 10 in which themeans for rotating each of the rotors comprises a separate shaftrotatably supported on the housing and projecting through a wall of thehousing so as to be accessible from outside the housing.
 12. A variableresistor in accordance with claim 11 in which the one rotor has aplurality of circumferentially spaced notches in its outer edge, each ofsaid notches being in radial alignment with a separate one of thecontact pads on the one rotor, and a detent spring member is mounted inthe housing and extends into one of the notches in the one rotor, saiddetent spring member being adapted to move out of and into the notchesas the one rotor is rotated.
 13. A variable resistor in accordance withclaim 12 in which the housing is rectangular having front, back, top,bottom and side walls, the shafts extend in spaced parallel relationbetween and are supported on the front and back walls and projectthrough the front wall of the of the housing, and each of the rotors ismounted on a separate one of the shafts.
 14. A variable resistor inaccordance with claim 13 in which the resistance path on the one rotoris on the front surface of the one rotor, the one contact is mounted onthe front wall of the housing with its terminal extending downwardlythrough the front wall and projecting beyond the bottom wall of thehousing, the resistance path on the other rotor is on the back surfaceof the rotor and the other contact is mounted on the back wall with itsterminal extending downwardly through the back wall and projectingbeyond the bottom wall of the housing.
 15. A variable resistor inaccordance with claim 11 including stop means for limiting the rotationof each of the rotors to the circumferential length of the resistancepath on the rotor.
 16. A variable resistor comprising a hollow housing;a pair of rotors rotatably mounted in said housing; means for rotatingeach of said rotors from outside of said housing; a separate resistancepath on each of said rotors, each of said resistance paths being a filmof a resistance material coated on the rotor, the resistance path on oneof the rotors being of a high resistivity resistanCe material so as toprovide large incremental changes in resistance along its length and theresistance path on the other rotor being of lower resistivity materialso as to provide small incremental changes in resistance along itslength; a pair of contacts within said housing, each of said contactshaving a terminal extending through and projecting beyond a wall of saidhousing; means electrically connecting each of said contacts to aseparate one of said resistance path; and means electrically connectingsaid resistance path to each other; each of the rotors being a flatcircular disk of an electrical insulating material and the resistancepaths being annular with spaced ends and extending circumferentiallyaround a flat surface of their respective rotors; the one rotor having aplurality of circumferentially spaced contact pads of an electricallyconductive material on the same surface as the resistance path and aseparate connecting strip of an electrically conductive materialextending from each of said contact pads to the resistance path, saidconnecting strips contacting said resistance path at uniformly spacedpoints along the resistance path so as to divide said resistance pathinto incremental lengths of uniform resistance; the resistance value ofthe total length of the resistance path on the other rotor beingsubstantially equal to the resistance value of each incremental lengthof the resistance path on the one rotor; each of the contacts being ametal strip having an arm extending from one end, fingers on the end ofthe arm, and the terminal extending from the other end, the fingers ofeach of the contacts slidably engaging a separate one of the rotors;each of the rotors having one the same surface as the resistance path acontact path of an electrical conductive material and a terminationstrip of an electrically conductive material extending between thecontact path and one end of the resistance path, and the fingers of eachof the contacts engaging a separate one of the contact paths; the meanselectrically connecting the resistance paths comprising a connectingstrip of an electrically conductive metal mounted in the housing andhaving a pair of arms extending therefrom with fingers on the ends ofthe arms, the fingers on one of the arms slidably engaging the one rotorso as to contact the contact pads and connecting strips as the rotor isrotated, and the fingers on the other arm slidably engaging theresistance path on the other rotor; the means for rotating the rotorscomprising a shaft secured to the other rotor and extending through awall of the housing so as to be accessible from outside the housing, anddrive means between the shaft and the one rotor so that the one rotor isrotated a circumferential distance equal to the circumferential lengthof each increment of the resistance path on the one rotor each time theshaft is rotated through a revolution; the drive means between the shaftand the rotor comprising a plurality of circumferentially spaced notchesin the edge of the one rotor, each of said notches being in radialalignment with a separate one of the contact pads on the one rotor, anda drive lug extending radially from the shaft, said drive lug fittinginto and driving the one rotor during a portion of each revolution ofrotation of the shaft.
 17. A variable resistor in accordance with claim16 in which the one rotor has a circumferentially elongated groove inits edge extending along the space between the ends of the resistancepath on the one rotor and between the notches at the ends of theresistance path.
 18. A variable resistor in accordance with claim 17including a hub shaft secured to the one rotor and a bearing hub on thesame wall of the housing that the shaft extends through, said bearinghub rotatably supporting the hub shaft so that the rotors rotate aboutspaced parallel axes.
 19. A variable resistor in accordance with claim18 in which the hub shaft has a plurality of circumferentially spacednotches in its outeR surface with the bottom of each notch being inradial alignment with a contact pad on the one rotor, and a springmember fits in a notch in the hub shaft and is adapted to move from onenotch to the next as the hub shaft is rotated.